PROJECT SUMMARY The mechanisms responsible for neural dysfunction in many neurodevelopmental disorders are challenging to address for many reasons. The human brain is relatively inaccessible for direct evaluation, human autopsy and imaging studies provide limited opportunities to study mechanism, animal models sometimes do not replicate key features, and immortalized neuron-like cell models have uncertain value for studying normal developmental processes. Induced pluripotent stem cells (iPSCs) provide a powerful new experimental tool to address these limitations, because they can be created from patients with defined mutations, and they can be differentiated in vitro into specific cell lineages to provide species-specific and lineage-specific cell models. In the current proposal we describe the development and characterization of a resource of iPSCs to serve as experimental cell-based models for Lesch-Nyhan disease (LND) and its attenuated variants. These disorders are caused by mutations in the HPRT1 gene, resulting in deficiency of the purine salvage enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt). LND provides an exceptionally tractable Mendelian disorder for iPSC modeling. Because this technology has not yet been methodically exploited for this disorder, the main focus of the current proposal is the development and initial characterization of a resource of iPSCs for discovery of novel biological processes and therapeutic interventions. Specifically, we will establish a well- characterized and clinically annotated bank of iPSCs from patients with a spectrum of disease severity as well as isogenic lines in which specific mutations have been introduced via gene editing methods (Aim 1); characterize morphological defects associated with differentiation of these iPSCs into neurons (Aim 2); determine the molecular and biochemical pathways affected in these cells using comprehensive transcriptomics, proteomics and metabolomics (Aim 3); and determine how early in the developmental process HGprt must be restored to correct neuronal defects (Aim 4). This project will result in a novel and critically important resource for future studies of the pathogenesis of LND and future studies of pharmacological or genetic therapeutic interventions.